Apparatus and method for intra-cardiac mapping and ablation

ABSTRACT

An intra-cardiac mapping system is based on locating the ports through which blood flows in or out the heart chambers. For many procedures, such as ablation to cure atrial fibrillation, locating the pulmonary veins and the mitral valve accurately allows to perform a Maze procedure. The location of the ports and valves is based on using the convective cooling effect of the blood flow. The mapping can be performed by a catheter-deployed expandable net or a scanning catheter. The same net or catheter can also perform the ablation procedure.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of prior U.S. patent application Ser.No. 15/697,744, filed Sep. 7, 2017, which is a continuation of priorU.S. patent application Ser. No. 14/804,810, filed Jul. 21, 2015, nowU.S. Pat. No. 9,987,083 issued on Jun. 5, 2018, which is a continuationof prior U.S. patent application Ser. No. 13/785,931, filed Mar. 5,2013, now U.S. Pat. No. 9,119,633, issued on Sep. 1, 2015, which is acontinuation-in-part of prior U.S. patent application Ser. No.11/475,950, filed Jun. 28, 2006, now U.S. Pat. No. 8,920,411, issued onDec. 30, 2014, the entire disclosure of each of these applications ishereby incorporated herein by reference.

TECHNICAL FIELD

This disclosure generally relates to minimally invasive heart surgery,also known as percutaneous cardiac surgery and particularly relates topercutaneous mapping and ablation.

BACKGROUND

Atrial fibrillation is a well known disorder in which spuriouselectrical signals cause an irregular heart beat. The disorder has awell known cure known as the Maze procedure, in which a border isablated around the sources of the spurious signals, typically in theleft atrium but sometimes in the right atrium. The procedure is verycommonly performed under direct vision, but difficult to performpercutaneously via a catheter because of the associated risk. Any errorin navigation inside the heart can cause fatal damage. The key to apercutaneous procedure is mapping of the inside of the right and leftatrium. Access to the right atrium is simple via the superior vena cava;the left atrium can be reached i) by perforating the transatrial septum,ii) via the aorta and the left ventricle or iii) via the pulmonaryveins.

Prior approaches to map the inside of the atrium relied on electricalactivity picked up from the atrium wall. These approaches requireintimate electrical contact, not always possible because of scar tissueand deposits. These approaches may fail to accurately map the edges ofthe openings where the veins enter the atrium; information that isuseful for correct placement of the ablation pattern. Other mappingmethods, such as using an array of ultrasonic transducers, are notpractical since such arrays typically will not fit through a catheter ofa reasonable size (8-10 mm diameter). A superior mapping apparatus andmethod, that enables safe execution of the Maze and other intra-cardiacprocedures is desirable.

A good survey article on the subject is: “Ablation of AtrialFibrillation: Energy Sources and Navigation Tools: A survey” by RuedigerBecker and Wolfgang Schoels (J. of Electrocardiology, Vol 37, 2004, pp55-61). The article includes an extensive bibliography.

SUMMARY

Embodiments of an intra-cardiac mapping system are based on locatingopenings or ports and values through which blood flows in or out of theheart chambers. For many procedures, such as ablation to cure atrialfibrillation, accurately locating the pulmonary veins and the mitralvalve allows performance of a Maze procedure. The openings, ports andvalves may be located based on the convective cooling effect of theblood flow. The mapping can be performed by a catheter-deployedexpandable net or a scanning catheter. The same net or catheter can alsoperform the ablation procedure.

In one embodiment, a method for intra-cardiac mapping comprises:introducing a plurality of flow sensors into an intra-cardiac cavity:locating points in a wall forming said cavity based on sensing bloodflow; and mapping said walls of said cavity based on said points. Themethod for intra-cardiac mapping may include said blood flow beingsensed by its convective cooling effect on a heated sensor. The methodfor intra-cardiac mapping may include said sensing being done by asteerable linear array. The method for intra-cardiac mapping may includesaid mapping being used for treating atrial fibrillation by RF ablation.The method for intra-cardiac mapping may include being used for treatingatrial fibrillation by microwave ablation. The method for intra-cardiacmapping may include said mapping being used for treating atrialfibrillation by cryogenic ablation. The method for intra-cardiac mappingmay include said mapping being used for treating atrial fibrillation bylaser ablation. The method for intra-cardiac mapping may include saidblood flow being sensed by the resistance change of a heated resistivewire.

In another embodiment, a method for intra-cardiac mapping comprises:introducing an expandable sensing mesh into said cavity via a catheter;using said mesh to locate openings in walls forming said cavity based onthe convective heat transfer of blood flowing through said holes; andmapping inside of said cavity based on location of said openings. Themethod for intra-cardiac mapping may include said blood flow beingsensed by its convective cooling effect on a heated sensor. The methodfor intra-cardiac mapping may include said sensing being done by asteerable linear array. The method for intra-cardiac mapping may includesaid mapping being used for treating atrial fibrillation by RF ablation.The method for intra-cardiac mapping may include said mapping being usedfor treating atrial fibrillation by microwave ablation. The method forintra-cardiac mapping may include said mapping being used for treatingatrial fibrillation by cryogenic ablation. The method for intra-cardiacmapping may include said mapping being used for treating atrialfibrillation by laser ablation. The method for intra-cardiac mapping mayinclude said blood flow being sensed by the resistance change of aheated resistive wire. The method for intra-cardiac mapping may includesaid mesh comprising small coils of nickel wire wound on a mesh of aflexible insulator. The method for intra-cardiac mapping may include anelectronic switch used to minimize the number of electrical wirespassing through said catheter.

In yet another embodiment, a method for treating atrial fibrillationcomprises: introducing at least one flow sensor into an intra-cardiaccavity; locating points in a wall forming said cavity based on sensingblood flow; mapping walls of said cavity based on said points; andablating a pattern into walls of said cavity based on said mapping. Themethod for treating atrial fibrillation may include said blood flowbeing sensed by its convective cooling effect on a heated sensor. Themethod for treating atrial fibrillation may include said sensing beingdone by a steerable linear array. The method for treating atrialfibrillation may include said mapping being used for treating atrialfibrillation by RF ablation. The method for treating atrial fibrillationmay include said mapping being used for treating atrial fibrillation bymicrowave ablation. The method for treating atrial fibrillation mayinclude said mapping being used for treating atrial fibrillation bycryogenic ablation. The method for treating atrial fibrillation mayinclude said mapping being used for treating atrial fibrillation bylaser ablation. The method for treating atrial fibrillation may includesaid blood flow being sensed by the resistance change of a heatedresistive wire. The method for treating atrial fibrillation may includesaid flow sensors also acting as electrodes for said ablation. Themethod for treating atrial fibrillation may include said flow sensorbeing based on temperature sensing and a same sensor being used tomonitor temperature during said ablation. The method for treating atrialfibrillation may include said ablation being unipolar. The method fortreating atrial fibrillation may include said ablation being bipolar.The method for treating atrial fibrillation may include said ablatedpattern being a Maze procedure.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, identical reference numbers identify similar elementsor acts. It is to be understood that the attached drawings are forpurposes of illustrating the concepts of the invention and may not be toscale. For example, the sizes, relative positions, shapes, and angles ofor associated with elements in the drawings are not necessarily drawn toscale, and some elements may be arbitrarily enlarged and positioned toimprove drawing legibility. Further, the particular shapes of theelements as drawn may differ from their actual shapes and, in thisregard, may be selected instead of the respective actual shapes for easeof recognition in the drawings.

FIG. 1 is a cross sectional view of the heart showing the mapping meshdeployed in the left atrium.

FIG. 2 is a cross sectional view of the sensing device.

FIGS. 3A and 3B are isometric views of the mesh in both folded andexpanded position.

FIG. 4 is an isometric enlarged view of a portion of the mesh.

FIG. 5 is an electrical schematic of a mapping and ablation system.

FIG. 6 is an electrical schematic of a simplified mapping system.

FIG. 7 is a schematic view of the display console of the system.

FIGS. 8A and 8B are graphical views of a mapping that illustrate aninterpolation principle.

FIG. 9 is a cross sectional view of an alternate embodiment, usingmechanical or manual scanning in one axis.

FIG. 10 is a cross sectional view of an alternate embodiment, usingmechanical scanning in two dimensions.

FIG. 11 shows the use of the invention for bipolar ablation.

DETAILED DESCRIPTION

In the following description, certain specific details are set forth inorder to provide a thorough understanding of various disclosedembodiments. However, one skilled in the relevant art will recognizethat embodiments may be practiced without one or more of these specificdetails, or with other methods, components, materials, etc. In otherinstances, well-known structures associated with apparatuses and methodsfor intra-cardiac mapping and ablation have not been shown or describedin detail to avoid unnecessarily obscuring descriptions of theembodiments.

Unless the context requires otherwise, throughout the specification andclaims which follow, the word “comprise” and variations thereof, suchas, “comprises” and “comprising” are to be construed in an open,inclusive sense, that is as “including, but not limited to.”

Reference throughout this specification to “one embodiment” or “anembodiment” means that a particular feature, structure or characteristicdescribed in connection with the embodiment is included in at least oneembodiment. Thus, the appearances of the phrases “in one embodiment” or“in an embodiment” in various places throughout this specification arenot necessarily all referring to the same embodiment. Furthermore, theparticular features, structures, or characteristics may be combined inany suitable manner in one or more embodiments.

As used in this specification and the appended claims, the singularforms “a,” “an,” and “the” include plural referents unless the contentclearly dictates otherwise. It should also be noted that the term “or”is generally employed in its non-exclusive sense including “and/or”unless the content clearly dictates otherwise.

The headings and Abstract of the Disclosure provided herein are forconvenience only and do not interpret the scope or meaning of theembodiments.

FIG. 1 shows a sensing and ablation mesh 7 inserted into a left atrium 3of a heart 1 according to one illustrated embodiment.

By way of example, the mesh 7 may be delivered via a catheter 60inserted via a superior vena cava 4 and penetrating a transatrial septumfrom a right atrium 2 of the heart 1. The mesh 7 is communicativelycoupled to the rest of the system, for example, by electrical wires 6.

Before any ablation takes place, the inside of the left atrium 3 ismapped in order to locate the openings or ports 8 leading to thepulmonary veins 5, as well as the mitral valve 9. A typical Mazeprocedure ablates a “fence” around openings or ports 8 to stoppropagation of spurious electrical signals which cause the heart 1 tocontract at the wrong times.

The mapping may locate some or all of the openings or ports 8 throughwhich blood flows in and out of the left atrium 3, as the Maze procedureis mainly concerned with the location of these openings or ports 8. Bythe way of example, in the left atrium 3, the four openings or ports 8leading to the pulmonary veins 5 as well as the mitral valve 9 may belocated. The location of these openings or ports 8 may be based on thefact that the convective cooling effect of the blood is significant, anda slightly heated mesh 7 pressed against the walls of the left and/orright atrium 3, 2 will be cooler at the areas which are spanning theopenings or ports 8 carrying blood.

FIG. 2 shows the ablation mesh 7 covered by miniature heating and/ortemperature sensing elements 10 a-10 c flow (collectively 10, only threeillustrated in the figure). Each one of these elements 10 a-10 ccomprises a few turns of a resistive wire, for example nickel wire,wound on an electrically insulated mesh. A low current is passed througheach element 10, raising a temperature of the element 10 by about 1degree C. above normal blood temperature. A first element 10 b, whichlies across an opening or port 8 of one of the pulmonary veins 5, willbe cooled by blood flow. The other elements are against a wall 3 andhence do not lie across any of the openings or ports 8.

By identifying the relatively cooler elements 10 a, 10 c on the mesh 7,the location of the openings or ports 8 may be found.

This method does not require intimate contact with the wall 3, as thecooling effect is significant even a few millimeters away from theopening.

The same elements 10 can be used as ablation electrodes during anablation stage. It was found that the power required to raise thetemperature of the mesh 7 by a small but easily detectable amount isvery small, on the order of 10-50 mW per element 10. If the elements 10are made of a material that has a significant change in resistance withtemperature, the temperature drop can be sensed by measuring a voltageacross the element 10 when driven by a constant current. A good choicefor element material is nickel wire, which is inert, highly resistiveand has a significant temperature coefficient of resistance (about 0.6%per deg C.). Since the resistance of the elements 10 is low (typically0.1-1 ohm), the electrical noise is very low and temperature changes aslow as 0.1 deg can be easily detected. For even higher detectionsensitivity, the voltage waveform can be sampled in sychronization withthe heart rate or the average voltage removed and only the changeamplified. Such methods are referred to as “AC coupling”. A furtherrefinement to reduce the electrical noise is to pass the signal througha digital band pass filter having a center frequency tracking the heartrate. To avoid any potential tissue damage, the temperature of theelements 10 of the mesh 7 is only slightly above the blood temperature,typically 0.1-3 degrees C. above blood temperature.

FIG. 3A shows the mesh 7 in a compressed configuration “A” and FIG. 3Bshows the mesh 7 in an expanded configuration “B”. Since the mesh 7 hasto fit into a catheter 60, the mesh 7 should be very flexible. Besideselements 10 discussed earlier, there is also a large number of leads 13coming out of the mesh 7. Leads 13 can be loose, as shown in FIG. 3B, ormay be bonded to the mesh 7. To avoid feeding a large number of wiresall the way to an operating console, an electronic selector switch maybe employed, which may, for example, be mounted in the catheter 60. Thisreduces the number of electrical wires from over 100 to about 10. Themesh 7 can be self-expanding (elastic) or balloon-expandable. Selfexpanding allows normal blood flow during the procedure. For balloonexpandable devices, the expansion balloon should be removed before themapping, to avoid blocking the flow of blood.

FIG. 4 shows the mesh 7 in more detail. Insulated longitudinal (i.e.,parallel to catheter) wires 25 are crossed by cross wires 26. Eachsection of the mesh 7 is covered by a few turns of thin (0.05-0.2 mm)nickel wire 10 having leads 13. The leads 13 can be regular thin copperwire. The longitudinal wires 25 can be stiffer than the cross wires 26,therefore can be made self-expanding by incorporating a core 14 made ofcoiled flexible metal wire such as Nitinol. A metallic core mayinterfere with the ablation process at higher frequencies and can bereplaced by simply making the longitudinal wires 25 of a polymericmaterial thicker than the cross wires 26. The cross wires 26, which mayform rings around wires 25, should be very flexible to compress into thecatheter 60. The cross wires 26 could incorporate a very thin wire orcoiled up wire. Use of a flexible mesh 7 not only allows percutaneousdelivery, but also permits the mesh 7 to follow the atrial volume changeeach heartbeat. The mesh 7 should stay in contact with or close to theatrial wall during the cardiac cycle, otherwise the measurement and theablation may only be performed during parts of the cardiac cycle. Thediameter of the longitudinal wires 25 and cross wires 26 are typically0.2-1 mm. The mesh 7 may include about 10-20 longitudinal wires 25 andabout 10-20 cross wires 26. The insulation can be any polymeric materialsuch as thin enamel or polymer coating. Practically any polymer can beused, as the maximum temperature it will be subject to, including duringthe ablation phase, is around 100 degrees C.

FIG. 5 shows an electrical system, according to one illustratedembodiment. The elements 10 may be resistive heaters wound on the mesh7. Each of the elements 10 is connected by electronic element switches15 (typically FET or MOS-FET type) to a single pair of wires leading outof the body to a mode selection switch 17. Element switches 15 areselected by de-multiplexer or selector 16. The de-multiplexer orselector 16 is controlled by a small number of wires or even a singlewire if data is sent in serial form, by a multiplexer 22. Elementswitches 15 and de-multiplexer or selector 16 may be built into thecatheter 60, which may, for example, be located near the point ofdeployment of the mesh 7. The element switches 15 have to carrysignificant power during the ablation phase.

The mode selection 17 selects between a mapping mode (position shown inthe drawing) and an ablation mode (second position of switch). In themapping mode, a current is created by a voltage source 18 and resistor19 (e.g., forming a constant current source) and routed into a selectedelement 10 by the element switches 15. For each measurement, the twoelement switches 15 that are connected to the scanned element 10 are inan enabled state (ON), the rest of the element switches being in adisabled state (OFF). The voltage drop across an element 10 is measuredby an analog to digital (A/D) converter 20 and fed to a control computer23. For greater accuracy, four terminal sensing can be employed. In apreferred embodiment, the detection is AC coupled, therefore the DCvoltage drops along the wires are of no consequence, and nofour-terminal sensing is needed. For AC coupling, the control computer23 may include a 0.5 Hz low pass filter, which may be implemented insoftware. The slight disadvantage of the AC coupled method approach isspeed, as the low signal frequency (e.g., about 1 Hz), requires a fewseconds per measurement. Other temperature sensors and/or approaches,such as thermistors or thermocouples, can be used in conjunction withthe elements 10. Mapping is achieved by turning on all of the elements10 (e.g., sequentially) and measuring the temperature of each. A map maybe formed in the control computer 23 and the lower temperature spots onthe mesh correspond to the openings or ports 8 leading to the veins orvalves.

When the mode selection switch 17 is in the ablation mode, a generator21 (e.g., Radio Frequency (RF)) is connected (e.g., sequentially) toselected elements 10 by the control computer 23 addressing themultiplexer 22 which controls the element switches 15 via thede-multiplex selector 16. The complete operation, including scanning andablation, can be completed in less than 5 minutes. The configurationillustrated in FIG. 5 implies unipolar ablation; however bipolarablation can be used as well and is discussed below. Clearly othersources of ablation can be used besides RF. Frequencies from DC tomicrowaves can be used, as well as delivery of laser power via opticalfibers or cryogenics via thin tubes. For laser ablation element switches15 are optical switches, while for cryogenic ablation the elementswitches 15 are valves, and in some embodiments may take the form ofheated elements such as resistive wires.

During ablation it is desirable to monitor the temperature of the modeselection switch 17 to the mapping position several times during theablation procedure. The measured temperatures can be displayed on adisplay 32 (FIG. 7). RF ablation is typically performed at frequenciesof 100 KHz-1 MHz and power levels which depend on the size of theelements 10, but can be as high as 100 W. Various RF ablation techniquesand equipment are well known in the art.

FIG. 6 shows an embodiment in which the mapping system is separate fromthe ablation system. In this system, the mesh 7 has very few connectingwires. As illustrated, each longitudinal wire 25 has a single outputwire and each cross wire 26 has a single output wire 13. For a 10×10mesh 7 with 100 nodes, only twenty-one wires are needed (ten plus tenplus ground wire), instead of two hundred wires. This allows all wiresto be brought directly out of the catheter 60. This also allowsplacement of selector switches 16 and 24 together with the controlsystem. For example, if the element marked as “A” is selected; a currentis selected to run through the longitudinal wire 25 which includeselement A. The voltage drop is sensed by the two circumferential wires13 that connect directly to A. Since no current flows in the otherelements at the time of measurement, the voltage drop is only caused byelement A. It is sensed by A/D converter 20 via double pole selector 24.

After a map is established, it is displayed on a display screen 32 asshown in FIG. 7. The surgeon can select which elements 10 will causetissue ablation in the atrium. The pattern formed is along the line ofthe standard Maze procedure. The location of the pulmonary veins 5 andthe mitral valve 9 is inferred from the temperature date and drawn onthe display screen.

FIGS. 8A and 8B demonstrate the principle of accurate location of theveins and valves even if the grid is relatively coarse. The exactlocation can be interpolated based on the fact that when only part ofthe element 10 is exposed to the blood flow. By the way of example, ifthe temperature of the mesh 7 is 1 degree C. above blood temperature andequals the blood temperature under normal blood flow (this wasexperimentally verified), the temperatures of a group of elements 10will be as shown in FIG. 8A when aligned with the opening or port 8 ofvein 5. The number near each element 10 is the temperature drop. Whenmoved, some of the elements 10 will only be partially positioned in theflow path under vein 5, as shown by FIG. 8B. The temperatures of thoseelements 10 will be between 0 and 1 degree above blood temperature. Theexact temperature drop between 0 to 1 corresponds with the exact shift.This allows accurate determination of the location and size of eachopening or port 8, data used by the control computer 23 to draw the mapshown in FIG. 7. A grid spacing of 10 mm allows about 1 mm accuracy.

An alternative to a full mesh is a partial mesh, or even a singlesensor, that is mechanically scanned across the area to be mapped. FIG.9 shows a linear sensor array 27 pushed into the atrium 2 via vein 4 bythe catheter 60. The linear sensor array 27 has a linear array ofelements 10 similar to those used in the full mesh 7. After a linearmapping is performed the linear sensor array 27 is rotated (as shown bybroken line 27′) a small amount (10-20 degrees) by stem 11 (similar toelectrical wires 6) and a new scan is performed. The same procedurespreviously described may be used for ablation.

FIG. 10 shows the use of a single steerable catheter 28 as a mapping andablation tool. Steerable catheters are controlled remotely bymechanical, magnetic, hydraulic or other means. A steerable catheter 28can be used to scan the inside of the atrium 3 by bending, as shown inbroken line 28′. The location is monitored by external or internalsensors. A position of a tip of the steerable catheter 28 can also bemonitored by fluoroscopy. The catheter tip contains a heating and/orablation element 10. Steerable catheters 28 may advantageously carry awide range of ablation systems, since only one connection and one pointis needed.

A full mesh trades a higher complexity for better speed and accuracywhen compared to linear arrays or single point scanning.

The previous examples were of unipolar ablation, with the ablationcurrent returning to ground via the patient's body. The disclosed systemcan also be used for bipolar ablation as shown in FIG. 11. In unipolarablation the same voltage is connected to both leads 13 and 13′ of anelement 10. In bipolar abalation the voltage is connected to lead 13while the other end, 13′, is grounded. It is important that the element10 will be of sufficient resistance to cause most of the ablationcurrent to flow through heart tissue 1. Electrodes 30 make contact withtissue 1 while the wire used in the element 10 is covered by aninsulator. The advantage of bipolar ablation is better control ofablation depth. Typical ablation temperatures are 60-80 degrees C. At ahigher temperature the tissue 1 becomes less conductive, forcing theablation current to seek a new path. This promotes full ablation of thetissue 1. The element 10 can also be designed to assist ablation bycreating heat when ablation voltage is applied across it.

One possible advantage of at least some of the presently disclosedembodiments over electrical potential mapping methods is that thepresently disclosed embodiments do not require perfect contact betweenthe mesh 7 and the tissue 1. The presently disclosed embodiments mayalso advantageously be less sensitive to the surface properties of thetissue, such as scar tissue or plaque.

If the mesh is separated from the tissue by a thin layer of blood, boththe temperature sensing and the ablation functions of the presentlydisclosed embodiments will still function properly.

The word “element” in this disclosure has to be interpreted in a broadsense as any element capable of sensing blood flow. Clearly the elementsdo not need to be heaters, as cooling elements will work equally well.If a material is injected into the blood flow, any sensor capable ofdetecting this material can be used to detect blood flow. By the way ofexample, if the blood is cooled or warmed slightly before returning tothe heart only temperatures sensors are needed. Since temperaturedifferences as low as 0.1 degree C. can be detected reliably, it isfairly simple to heat or cool the blood slightly before it returns tothe heart (even by a simple external pad).

The above description of illustrated embodiments, including what isdescribed in the Abstract, is not intended to be exhaustive or to limitthe embodiments to the precise forms disclosed. Although specificembodiments of and examples are described herein for illustrativepurposes, various equivalent modifications can be made without departingfrom the spirit and scope of the disclosure, as will be recognized bythose skilled in the relevant art.

The various embodiments described above can be combined to providefurther embodiments. All of the U.S. patents. U.S. patent applicationpublications, U.S. patent applications, foreign patents, foreign patentapplications and non-patent publications referred to in thisspecification and/or listed in the Application Data Sheet areincorporated herein by reference, in their entirety. Aspects of theembodiments can be modified, if necessary, to employ systems, circuitsand concepts of the various patents, applications and publications toprovide yet further embodiments.

These and other changes can be made to the embodiments in light of theabove-detailed description. In general. in the following claims, theterms used should not be construed to limit the claims to the specificembodiments disclosed in the specification and the claims, but should beconstrued to include all possible embodiments along with the full scopeof equivalents to which such claims are entitled. Accordingly, theclaims are not limited by the disclosure.

1. (canceled)
 2. A medical system comprising: a catheter comprising: aplurality of resistive elements located on a distal end portion of thecatheter, the plurality of resistive elements electrically connectedserially to one another, each resistive element of the plurality ofresistive elements configured to raise a temperature of the resistiveelement in response to passage of electrical current through theresistive element; a first pair of leads electrically connected to afirst resistive element of the plurality of resistive elements tofacilitate sensing of a voltage drop across the first resistive elementin response to passage of electrical current through the first resistiveelement; and a second pair of leads electrically connected to a secondresistive element of the plurality of resistive elements to facilitatesensing of a voltage drop across the second resistive element inresponse to passage of electrical current through the second resistiveelement, wherein the first pair of leads and the second pair of leadsshare a same lead, and wherein the first resistive element and thesecond resistive element are electrically connected in series adjacentone another.
 3. The medical system of claim 2, comprising an electricalcurrent source configured to supply electrical current to the pluralityof resistive elements.
 4. The medical system of claim 2, comprising acontrol computer coupled at least to the plurality of the resistiveelements and configured at least to determine (a) temperature at leastproximate the first resistive element based at least on the sensedvoltage drop across the first resistive element, (b) temperature atleast proximate the second resistive element based at least on thesensed voltage drop across the second resistive element, or (c) both (a)and (b).
 5. The medical system of claim 4, wherein the control computeris configured to cause a display device to display information based atleast on (a), (b) or both (a) and (b).
 6. The medical system of claim 5,wherein the displayed information comprises information visuallyindicating changes in temperature.
 7. The medical system of claim 6,wherein the information visually indicating changes in temperature isdisplayed in an overlapping relationship with a map of at least part ofa chamber of a heart.
 8. The medical system of claim 6, wherein theinformation visually indicating changes in temperature comprises (i)information indicating a difference in temperature between thedetermined temperature at least proximate the first resistive elementand a first temperature that is above normal blood temperature, butbelow a temperature that causes tissue damage, (ii) informationindicating a difference in temperature between the determinedtemperature at least proximate the second resistive element and a secondtemperature that is above normal blood temperature, but below atemperature that causes tissue damage, or (iii) both (i) and (ii). 9.The medical system of claim 6, wherein the information visuallyindicating changes in temperature comprises (i) information indicating adifference in temperature between the determined temperature at leastproximate the first resistive element and a first temperature that is0.1 to 3 degrees Celsius above normal blood temperature, (ii)information indicating a difference in temperature between thedetermined temperature at least proximate the second resistive elementand a second temperature that is 0.1 to 3 degrees Celsius above normalblood temperature, or (iii) both (i) and (ii).
 10. The medical system ofclaim 3, comprising a control computer coupled at least to theelectrical current source to cause the electrical current source tosupply electrical current to the plurality of resistive elements tocause each respective resistive element of at least some resistiveelements of the plurality of resistive elements to raise its respectivetemperature above normal blood temperature, but below a temperature thatcauses tissue damage.
 11. The medical system of claim 3, comprising acontrol computer coupled at least to the electrical current source tocause the electrical current source to supply electrical current to theplurality of resistive elements to cause each respective resistiveelement of at least some resistive elements of the plurality ofresistive elements to raise its respective temperature no more than 3degrees Celsius above normal blood temperature.
 12. The medical systemof claim 2, wherein the catheter comprises an ablation element set, eachablation element in the ablation element set configured to transmittissue ablative energy during an ablation operation employing theablation element set.
 13. The medical system of claim 12, wherein theablation operation comprises radio frequency (RF) ablation.
 14. Themedical system of claim 12, wherein the ablation operation comprisesmicrowave ablation.
 15. The medical system of claim 12, wherein theablation operation comprises laser ablation.
 16. The medical system ofclaim 12, wherein the ablation operation comprises cryogenic ablation.17. The medical system of claim 2, wherein the catheter comprises anexpandable structure, and wherein at least the plurality of resistiveelements is located on the expandable structure.
 18. The medical systemof claim 17, wherein the resistive elements of the plurality ofresistive elements are longitudinally arranged along the expandablestructure.
 19. The medical system of claim 17, wherein the expandablestructure is self-expandable.
 20. The medical system of claim 17,wherein the expandable structure comprises a balloon.
 21. The medicalsystem of claim 2, wherein the catheter comprises at least onetemperature sensing element configured to sense temperature at leastproximate a resistive element of the plurality of resistive elements.22. The medical system of claim 2, wherein the catheter comprises atleast one thermistor.
 23. The medical system of claim 2, wherein thecatheter comprises at least one thermocouple.